1. Field of the Invention
The present invention relates to a scanning optical apparatus and an image forming apparatus using the same. The present invention is suitable for an image forming apparatus such as a laser beam printer, a digital copying machine, or a multifunction printer, which adopts an electrophotography process.
2. Description of the Related Art
Up to now, a scanning optical apparatus is used for an image forming apparatus such as a laser beam printer (LBP), a digital copying machine, or a multifunction printer. In the scanning optical apparatus, a light beam optically modulated and emitted from a light source unit according to an image signal is periodically deflected by a deflector configured by, for example, a rotary polygon mirror. The deflected light beam is focused on a photosensitive recording medium (photosensitive drum) surface in the form of a spot by means of an imaging optical system (imaging lens system) having an fθ characteristic, and the surface is optically scanned to record an image.
In order to adjust a timing for starting the formation of the image on the photosensitive drum surface before scanning the photosensitive drum surface with the light spot, a synchronization detection sensor serving as an optical detector is provided for the scanning optical apparatus described above.
In order to increase a printing speed, a light source unit for emitting multiple light beams may be used for the scanning optical apparatus. In this case, when polarized directions of the respective light beams are not aligned, a ratio between a P-polarized component and an S-polarized component (polarized component ratio) of each of the light beams which passes through or is reflected on optical elements before reaching a surface to be scanned is changed among the respective light beams. As a result, the amount of light on the surface to be scanned is changed among the respective light beams. Various scanning optical apparatus which solve such a problem have been proposed up to now (see Japanese Patent Application Laid-Open No. 2001-337285 and Japanese Patent Application Laid-Open No. 2005-315997).
In Japanese Patent Application Laid-Open No. 2001-337285 and Japanese Patent Application Laid-Open No. 2005-315997, a P-polarized reflectance and an S-polarized reflectance for each of the optical elements which the light beams pass through or are reflected on are made equal to each other. Therefore, even when the polarized component ratio is changed relative to a design value, the unevenness of the amount of light on the surface to be scanned is suppressed.
In order to increase a processing speed, a high-sensitivity photosensitive drum may be used for an image pickup apparatus using the scanning optical apparatus. In this case, when the power of laser light emitted from a laser diode (LD) is reduced corresponding to the high-sensitivity photosensitive drum, there is a fear that light emitted from the laser diode is used in an unstable driving current region. Various scanning optical apparatus which solve such a problem have been proposed up to now (see Japanese Patent Application Laid-Open No. H10-78556).
In Japanese Patent Application Laid-Open No. H10-78556, a light amount reduction unit is provided on an optical path between the light source unit and the surface to be scanned. Therefore, while the laser diode is used in a stable driving current region, the amount of light on the surface to be scanned is reduced to use the high-sensitivity photosensitive drum.
In the scanning optical apparatus described in Japanese Patent Application Laid-Open No. 2005-315997, the following method is employed in order to suppress the unevenness of the amount of light on the surface to be scanned. In other words, the design is made such that an illuminance distribution on the surface to be scanned is uniform at an arbitrary image height in view of both a Fresnel reflection component of a transmission member located between a deflecting unit and the surface to be scanned and a reflecting film characteristic of a reflection member located between the deflecting unit and the surface to be scanned.
A scanning optical apparatus has been known in which a light beam entering a deflecting surface of the deflecting unit is caused to be obliquely entered thereon in a sub scanning direction (oblique incidence system) to make compact. In the scanning optical apparatus using the oblique incidence system, a light beam deflectively reflected on the deflecting surface of the deflecting unit travels at an angle in the sub scanning direction. As a result, in a case of an imaging lens located between the deflecting unit and the surface to be scanned, the light beam passes through a region except for a central portion in a height direction, of an effective portion of the imaging lens.
The light beam passing through the region except for the central portion in the height direction, of the effective portion of the imaging lens as described above is affected by birefringence caused by a residual stress of the imaging lens, and hence a polarized direction of the light beam changes as compared with a light beam passing through the region of the central portion. As a result, the ratio between the P-polarized component and the S-polarized component changes when the light beam passes through or is reflected on optical elements located after the imaging lens, resulting that the illuminance distribution on the surface to be scanned changes as compared with a case where the birefringence of the imaging lens is not taken into account.
Therefore, it is necessary to make the design such that the illuminance distribution on the surface to be scanned is uniform in view of the influence of birefringence of the transmission member in the polarized direction in addition to both the Fresnel reflection component of the transmission member and the reflecting film characteristic of the reflection member, which are located between the deflecting unit and the surface to be scanned.